Methods of reducing thermo-oxidation of polymers such as polyacrylonitrile

ABSTRACT

A filter comprising a polymer material comprising a polymer matrix such as polyacrylonitrile containing dispersed metal oxide particles. The metal oxide particles are for example ferric oxide particles, and the polymer material contains generally 0.25-3% by weight of these particles. The filters are resistant to thermo-oxidation and have reduced shrinkage or degradation.

FIELD OF THE INVENTION

The present invention relates to methods of reducing oxidation and/orshrinkage of polymers in high temperature environments. The presentinvention relates to a broad range of polymers, including acrylicpolymers.

BACKGROUND OF INVENTION

Polyacrylonitrile (PAN) is an acrylic polymer commonly used in manyforms. It is manufactured by polymerisation of acrylonitrile monomer(CH₂.CHCN) using the solution, suspension or emulsion methods.Typically, a small percentage of co-monomer (such as methacrylate—CH₂:CHCOOCH₃, or vinyl acetate —CH₂:CH—O—CO—CH₃) is incorporated intothe polymer chain to control crystallinity and hence to modify somephysical properties of the polymer. The amount of co-monomer in PANusually ranges from 0 to 15%. Generally PAN is considered to be ahomopolymer (ie: (CH2.CHCN)_(n)) when it contains less than 2%co-monomer.

Molecular weight of PAN can range from 10,000 to 500,000 or more and isclosely controlled within the polymerisation process as it has a strongeffect on both the efficiency of the polymer production process and thephysical properties of the end product.

The basic form of PAN is a fine white powder. In manufacturing PANmaterials, this powder is usually dissolved in a solvent (eg, dimethylformamide, dimethyl acetamide or water-based sodium rhodamide) and theresulting polymer solution (or “dope”) is either cast in the form of afilm or spun as a fine fibre. Fibres based on a copolymer PAN are usedfor a wide variety of textile applications—as knitwear or as a wovenfabric for clothing or home textiles. The fibrous form of homopolymerPAN is commonly used for manufacturing a wide variety of woven or feltedmaterials for technical end uses or as a reinforcing element incomposite materials (such as brake linings or concrete).

Because of its high level of chemical resistance, homopolymer PAN isalso used in various industrial processes and products. Filter materialsfor both dry and wet applications are commonly made out of PAN.

Fabric filters using PAN fibres for particle collection are seeingwidespread use for gas cleaning in many industrial processes. They havea high collection efficiency and produce a gas stream with a very lowlevel of particulates. Operating costs may, however, be considerable asthe filter bags must be replaced when either the pressure drop acrossthe filter, or the rate of bag failures, or the level of dust emissionsbecome excessive. The removal of particles of unburnt ash fromcoal-fired power station exhaust gases is one specific example.Depending on the design of the plant, such a filter material might haveto operate at temperatures of up to 135° C. in a flue gas environmentcontaining gaseous oxides of nitrogen and sulphur as well as water,carbon dioxide, nitrogen and oxygen. PAN filter material is well suitedto this purpose and has been widely employed in the filters of largepower stations of Australia, South Africa, and elsewhere.

One disadvantage of polyacrylonitrile is its susceptibility toshrinkage. The shrinkage of polyacrylonitrile and other polymers used inhigh extreme conditions (such as high temperature and oxidativeconditions) is generally a consequence of oxidation of the polymer. Evenin the reduced oxygen environment of combustion flue gas, the rate ofPAN oxidation is significant at temperatures above 115° C. Suchoxidation causes the PAN to shrink and to become brittle and losestrength. It frequently leads to premature physical failure of thefilter media under the combined effects of growing tensile forces andreduced tensile strength.

While it is possible to introduce standard organic antioxidants intoPAN, these are quickly decomposed or migrate out of the crystallinestructures in the polymer matrix at high temperatures.

Filters for use in high temperature environments are also made fromother polymer materials that are, to a greater or lesser degree, alsoprone to shrinkage due to oxidation. These include polyesters,polyamides, polyolefins such as polypropylene, polyaramids such asNomex™, fluorocarbon fibres such as polytetrafluoroethylene andpolyphenylene-based polymers such as polyphenylene sulphide.

SUMMARY OF INVENTION

According to one embodiment, the present invention provides a filtercomprising a polymer material comprising a polymer matrix containingdispersed metal oxide particles.

According to another embodiment, there is provided a use of a polymermaterial comprising a polymer matrix and a metal oxide in themanufacture of a filter.

According to a third embodiment, the present invention provides for theuse of a polymer material comprising:

-   -   a polymer matrix selected from polyacrylics, polyesters,        polyamides, polyolefins, polyaramids, fluorocarbon polymers and        polyphenylene polymers, including copolymers and derivatives        thereof, and mixtures thereof, and    -   a metal oxide,        in an environment having a temperature of at least 100° C.

An example of such an environment is a flue gas environment.Accordingly, the polymer material may be used in a component of flue gastreatment plant.

DETAILED DESCRIPTION OF THE INVENTION

The present applicants have surprisingly found that the inclusion ofmetal oxide in materials formed from polymeric materials such as filtersintended to be used in high temperature environments, protects thepolymer from oxidation (ie thermo-oxidation) and/or degradation, such asshrinkage.

The term filter is used in its broadest sense and encompasses a filterunit, as well as the filtering component of such units. This filteringcomponent is also referred to as the “filter material” or “filtermedia”.

The term metal oxide refers to the class of metal oxides having morethan one possible oxidation state. This therefore excludes the alkaliand alkaline earth metal oxides such as magnesium oxide, and encompassesthe transition metal oxides including, notably, zinc oxide, nickeloxide, iron oxide, copper oxide and cobalt oxide. A mixture of metaloxides or a mixed metal oxide may be used.

Iron oxide is suitably the metal oxide used. The iron oxide ispreferably ferric oxide (Fe₂O₃).

The metal oxide is present in the polymer matrix in particulate form.Although any particle size will have an affect on the preservation ofthe polymer, it has been found that the metal oxide is particularlyeffective in the form of fine particles. Such particles are preferablyless than 1 μm (10⁻⁶ metre) in size, and more preferably less than 500nm in size. For best effect, the particles may be as small as 10 nm(10⁻⁸ metre) in diameter.

Such particles can be produced standard ultra-fine grinding techniquesor variants thereof using standard equipment, such as a ball mill.Processes which maximise particle dispersion are advantageous to avoidclumping and improve homogeneity in the product. Some of the morerecently developed methods are suitable for forming the particles,including the following:

-   Gonsalves K. E., et al., Synthesis of Acicular Iron Oxide Particles    and their Dispersion in a Polymeric Matrix. J. Mater. Sci. 2001: 36:    2461-71.-   Janzen C., et al., Characteristics of Fe2O3 Nanoparticles from Doped    Low Pressure H2/O2/Ar Flames. J. Nanopart. Res. 1999: 1:163-7.-   Puntes V. F., et al., Synthesis of Colloidal Cobalt Nanoparticles    with Controlled Size and Shapes. Top. Catal. 2002: 19(2): 145-8.-   Xia B. I., et al., Novel Route to Nanoparticle Synthesis by    Salt-Assisted Aerosol Decomposition. Adv. Mater. 2001: 13(20):    1579-82.-   Mori Y., et al., Titanium Dioxide Nanoparticles Produced in    Water-In-Oil Emulsion. J. Nanopart. Res. 2001: 3: 219-25.

Unlike conventional organic antioxidants, these metal oxides, such asFe₂O₃, are stable at high temperatures and have been found not tomigrate out of the polymeric structure.

Greater concentrations and finer particle size dispersion of metaloxides increase the resistance of the polymer matrix to oxidation.

The amount of metal oxide in the polymer material should be an amountsufficient to reduce the tendency of the polymer to oxidise in hightemperature environments when compared with the same polymer notincluding the metal oxide.

Preferably, the metal oxide constitutes a maximum of 5% by weight of thepolymer material. At higher levels, the metal oxide can lead tobrittleness of the polymer material reducing the mechanical strength andelasticity. Preferably, the metal oxide constitutes a minimum of 0.1% byweight of the polymer material. The preferred range of inclusion of themetal oxide is from 0.25% to 3% by weight of the polymer material.

The polymer matrix is suitably chosen from one or more of the range ofpolymers which are prone to oxidation. This includes polyacrylics,polyesters, polyamides, polyolefins, fluorocarbon fibres andpolyphenylene polymers, including copolymers and derivatives thereof andmixtures thereof. References to “polymers” encompasses co-polymers,unless the context is to the contrary. The term “polyacrylics”encompasses mondacrylics, and any acrylic-containing polymers, notablythose based on acrylic acid and its esters and derivatives. The term“polyolefins” encompasses the broad range of vinyl polymers includingpolypropylene, polyvinyl esters, polyvinyl ethers, polyvinyl acetates,polystyrenes, and halogenated polyvinyls, including polyvinylidenechloride. An example of a polyaramid is Nomex™. Fluorocarbon polymersencompass those polymers containing fluorine atoms, such aspolytetrafluoroethylene. Such polymers may also be classified in etherpolymer classes such as polyolefins. Polyphenylene polymers encompasspolymers such as polyphenylene sulphide.

According to one embodiment, the polymer is apolyacrylonitrile-containing polymer, or a derivatives thereof such asmethacrylonitrile. The polymer may be polyacrylonitrile (PAN) or acopolymer of acrylonitrile with one or more other monomers. Suitablecomonomers include acrylics other than acrylonitrile (such asmethacrylonitrile); acrylamides and their derivatives (includingmethacrylamide); acrylic acid and its esters and derivatives thereof(including methacrylic acid and its esters); olefins and derivativesthereof including vinyl esters and ethers such as vinyl acetate, vinylbutyrate or vinylbutrylether, together with styrene and vinyl halogenssuch as vinyl chloride and vinylidene chloride; and maleimides.

In the situation where the polymer matrix is based on acrylonitrile, itis preferred that the polymer material comprises at least 85%acrylonitrile, more preferably at least 95% and most preferably at least99% acrylonitrile by weight. Preferably the co-monomer content rangesfrom 0 to 10%, and more preferably from 0 to 5% by weight of the polymermaterial. Preferably, the average molecular weight of the polymer isbetween 100,000 and 300,000.

Suitably, the components made from the polymer materials are in the formof filaments, fibres, yarns, webs, fabrics, mats, films or sheets, or acombination of these. The nature of these polymers is such that they aresuitable for production into fibres or yarns, which may then be woveninto flexible materials. Accordingly, they are most suitable forformation into flexible fabric-type filter media.

According to a further embodiment, the present invention provides amethod of forming a filter, the method comprising:

-   -   providing a polymer matrix;    -   incorporating particulate metal oxide into the polymer matrix;    -   casting or spinning the product into a film or a fibre; and    -   forming the film or fibre into a filter material.

In dispersing the metal oxide into the polymer melt or solution, thebest results are achieved when the individual particles of metal oxideare separated and distributed widely throughout the polymer matrix.

According to the present invention, there is provided a filter formed atleast partly from a polymer material comprising:

-   -   a polymer matrix, and    -   a metal oxide, the metal oxide being incorporated into the        polymer matrix in an amount to provide a polymer material that        has a characteristic shrinkage rate of 0.2% or less per 1000        test hours in a flue gas environment of approximately 125° C.

The shrinkage test used to calculate the “characteristic shrinkage rate”involves the following:

-   (i) Forming a yarn from the polymer having a breaking strength of    approximately 20 N (plus or minus 5 N);-   (ii) Positioning the yarn in a flue gas environment at 125° C. (plus    or minus 5° C.) and of approximately 7% oxygen content and under    light tension that is no more than 2% of the breaking strength of    the yarn;-   (iii) Measuring and recording the changes in the length of the yarn    at intervals of 24 hours for a period of up to 5,000 hours of flue    gas exposure; and-   (iv) Calculating the percentage change in length of the yarn between    two points in time separated by at least 500 hours and between which    the length of the yarn has varied at a measurable and approximately    constant rate. Preferably, the yarn length between the 4,000 hour    point and the 5,000 hour point will be taken as an ideal indicator    of the characteristic shrinkage rate of the polymer.

To comply with requirement (ii) set out above, the yarn is preferablypositioned in a duct or a filter chamber of a flue gas cleaning systemand subjected to the following steps:

-   (v) Attaching one end of a 5 metre length of the yarn to a fixed    mounting point on the floor of the duct or filter chamber; and-   (vi) Looping the free end of the yarn over a pulley above the fixed    end of the yarn, and tying an appropriately-sized weight to the free    end of the yarn to apply to the yarn a tensile force no greater than    2% of the breaking strength of the yarn.

The change in length of the yarn is preferably measured by:

-   (vii) Coupling the pulley to a rotary position sensor that will turn    freely as the length of the yarn changes to detect the change in    length of the yarn.

It will be understood that the pulley used in the test set out abovemust be capable of turning freely as the length of the yarn changes sothat the change in length of the yarn can be measured accurately, forexample up to 0.02% of the length. It will also be appreciated that thediameter of the pulley should be chosen so that the sum of thefrictional torques applied to the pulley shaft by its bearings, and bythe rotary transducer (part of the rotary position sensor) does notimpede the rotation of the pulley or significantly change the constanttension experienced by the yarn under the action of the tensioningweight.

The pulley and rotary position sensor must also be capable ofwithstanding the flue gas conditions for several thousand hours ofuninterrupted testing. Preferably the output signal from the sensor isconnected to an automatic data recording system for ease of testing andcalculation of the characteristic shrinkage rate of the polymer. Otherdetails concerning this test are set out in the Examples below.

In the aforementioned test for shrinkage under real operatingconditions, a shrinkage rate for polyacrylonitrile of 0.5% per 1,000test hours at 125° C. would be considered normal. To significantlyreduce or prevent shrinkage, the incorporation of a metal oxide additiveinto such a polymer in the manner of the invention would suitably reducethat rate to less than 0.2% per 1,000 test hours.

The present invention also provides a filter formed at least partly froma polymer material comprising:

-   -   a polymer matrix, and    -   a metal oxide, the metal oxide being incorporated into the        polymer matrix in an amount to provide a polymer material that        has a characteristic shrinkage rate that is 40% or less compared        to the characteristic shrinkage rate of a polymer material        without the metal oxide measured over a test period of 1000        hours in a flue gas environment at approximately 125° C.

Although shrinkage is one notable example of degradation that impacts onfilter performance, other forms of degradation are also evidence ofoxidation of the polymer, including cracking and reduction in tensilestrength.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described in further detail withreference to a non-limiting example of a polymer material of oneembodiment of the invention. For the assistance in understanding thebenefits of the polymer materials of the present invention, referencewill also be made to a number of drawings, in which:

FIG. 1 is a graph illustrating the shrinkage results for a polymer ofthe prior art compared with a polymer of a preferred embodiment of thepresent invention;

FIG. 2 illustrates FTIR spectra for the polymer materials of the priorart and of the preferred embodiment of the invention prior to and postexposure to flue gas in a power station; and

FIG. 3 is a graph comparing the results illustrated in FIG. 2 in anotherformat.

EXAMPLE 1

20 kg of a polymer containing 99.5% by weight acrylonitrile and 0.5% byweight of methylacrylate with a relative viscosity of 3.0 was dissolvedin 80 kg of dimethylformamide solvent at 90° C. 200 g of a fine ferricoxide (Fe₂O₃) powder with an average particle size of 200 nm was addedto the dope and mixed in well. The dope was pressed through a spinneretwith 5,000 holes of 60 μm diameter in a coagulation bath containing amixture of solvent and water. The filaments were washed with water,5-fold stretched in boiling water, treated with a fibre finish, dried onheated rollers, 2-fold stretched, heat-set, crimped and cut. Theresultant fibres had a red-brown colour and textile properties of 2.1dtex (decitex; corresponding to a diameter of approximately 15 μm aspolyacrylonitrile has a density of approx 1.18 g/cm³), 49 cN/tex(c=centi) and an elongation of 15%. The fibres were then spun into afolded yarn with a count of Nm 24/2, a strength of 29.5 cN/tex and anelongation of 13.5%. This yarn was then used in the production of afilter material.

EXAMPLE 2

A yarn was made by the method outlined in Example 1 so as to include aconcentration of 1% by weight ferric oxide (200 nm) in conventional PAN.This polymer was found to have a reduced tendency to shrink when exposedto a high temperature environment for an extended period of time, and toexhibit significantly less oxidation (by between 30% to 40%) whencompared to conventional PAN.

The tendency of the yarn of Example 2 to shrink was measured by the“characteristic shrinkage test” which is described in further detailbelow. The same yarn not including the metal oxide additive was alsosubjected to this test. FIG. 1 illustrates the results of this test. Itis noted that the two yarns were suspended under a light tension insidea filter chamber of a coal-fired power station where they were exposedto 125° C. flue gas (containing approximately 7% oxygen) for an initialperiod of more than 600 hours while their rate of shrinkage wascontinuously recorded. Ideally, this test would continue for severalthousand hours to accurately determine the shrinkage rates of the yarnsunder the test conditions. In FIG. 1, the shrinkage rate of the yarncontaining 1% Fe₂O₃ is seen to be negligible compared to that of theconventional yarn.

Further evidence of the effectiveness of this method for preservingpolyacrylonitrile can be drawn from infra-red spectroscopic analysis ofthe modified and conventional PAN both before and after exposure to thehigh temperature atmosphere of the power station flue gas. Such evidenceis presented in FIG. 2. The FTIR (Fourier Transform Infra-red)absorption spectra of the “new” (unexposed) conventional and modifiedyarns (the upper and lower charts respectively at the left-side of thefigure, labelled (a) and (b)) are compared with the FTIR spectra (theupper and lower charts respectively at the right-side of the figurelabelled (c) and (d)) taken after more than 1000 hours of exposure inthe power station flue gas atmosphere.

In these spectra, the peaks centred on 1700 cm⁻¹ wavenumber representcarboxyl and carbonyl functional groups arising from oxidation of thePAN. The peaks at for example 1450 and 2919 cm⁻¹ represent the CH₂groups of the polymer. Because these peaks are relatively unaffected bysuch oxidation, they can serve as reference point for assessing thelevels of absorbance. The strong peak corresponding to the nitrile groupof the PAN molecule at 2242 cm⁻¹ can also serve as a satisfactoryreference.

As shown in FIG. 3, a comparison of the ratios of oxidation to referencepeaks in the spectra of the two exposed yarns shows that degree ofoxidation of the modified polymer of Example 2 is between 60% to 70%that of the conventional yarn.

Greater concentrations and finer particle size dispersion of Fe₂O₃increase the resistance of PAN to oxidation.

Characteristic Shrinkage Rate Test

A test was devised to assess the susceptibility of polyacrylonitrile andother polymers to shrinkage under extreme conditions. This test involvesmeasuring the shrinkage rate, in a combustion flue gas environment, of ayarn formed from the polymer. This test is described in detail below.

The yarn to be tested preferably has a breaking strength ofapproximately 20 N and is of 5 metres (or greater) in length. In thetest, one end of the yarn is attached to a fixed mounting point on thefloor of a duct or filter chamber of the flue gas cleaning system. Fromthis mounting point, the yarn travels vertically upwards for most of itslength. At its upper reaches, the yarn is draped over a pulley and tiedto a small weight. This weight applies a small tensile force ofpreferably no more that 2% of the ultimate tensile strength of the yarn.The pulley is attached to the shaft of a rotary position sensor thatwill turn freely as the length of the yarn changes. The diameter of thepulley is chosen so that the sum of the frictional torques applied tothe pulley shaft by its bearings, and by the rotary transducer, does notimpede the rotation of the pulley or significantly change the constanttension experienced by the yarn under the action of the tensioningweight. The rotary position sensor is preferably chosen so that it canresolve a 0.02% change in the length of the yarn. It must also becapable of withstanding the flue gas conditions for several thousandhours of uninterrupted testing. The output signal from the sensor ispreferably connected to an automatic data recording system.

After being installed into the flue gas environment, the length of thetest yarn is recorded in the aforementioned manner at regular intervalsof preferably 24 hours or shorter over a test period that wouldpreferably extend for a period of at least 5,000 hours (approximatelyseven months) of flue gas exposure. The temperature of the flue gasenvironment is also regularly measured and recorded.

A record of the length of the test yarn and the temperature of itsenvironment is built up in this way and charted as a function of testtime.

Over an initial period of the test that might extend to 1,000 hours ormore, it is not uncommon to observe some degree of lengthening of thetest yarn due to relaxation of mechanical stresses in the polymer underthe applied tension. At a temperature of 125° C., such extension mightbe more than 1% of the length of the yarn. Beyond this point, however,the effect of thermo-oxidative degradation of the polymer will becomeapparent as the test yarn begins to shrink. At this point, it has beencommonly (but not exclusively) found that the rate of shrinkageincreases over the next thousand hours of the test as the effect ofmechanical relaxation subsides and thermally-driven chemical reactionsalter the structure of the polymer material. By the time an additional1,000 to 2,000 hours of testing (at a temperature of 125° C.) haveelapsed, it is commonly (but not exclusively) found that the rate ofshrinkage becomes approximately constant. When more than 5,000 hours oftotal test time has elapsed at a temperature of preferably 125° C. orhigher, an essentially constant rate of shrinkage is considered to be acharacteristic property of the test material under conditions of stressarising from the thermo-oxidative test environment.

In the aforementioned test for shrinkage under real operatingconditions, a shrinkage rate for polyacrylonitrile of 0.5% per 1,000test hours at 125° C. is considered normal. To significantly reduce orprevent shrinkage, the incorporation of a metal oxide additive into sucha polymer in the manner of this invention would reduce that rate topreferably less than 0.2% per 1,000 test hours.

The above examples serve to illustrate the principle of the inventiononly, and are not intended to limit the scope thereof. Variousmodifications can be made to the materials and methods described in theexamples without departing from the spirit and scope of the invention.

In the claims which follow and in the preceding description of theinvention, except where the context requires otherwise due to expresslanguage or necessary implication, the word “comprise” or variationssuch as “comprises” or “comprising” is used in an inclusive sense, i.e.to specify the presence of the stated features but not to preclude thepresence or addition of further features in various embodiments of theinvention

1. A filter comprising a polymer material comprising a polymer matrixcontaining dispersed metal oxide particles.
 2. The filter according toclaim 1, wherein the metal oxide is selected from the group consistingof zinc oxide, nickel oxide, iron oxide, copper oxide, cobalt oxide andmixtures thereof.
 3. The filter according to claim 2, wherein the metaloxide is iron oxide.
 4. The filter according to claim 3, wherein themetal oxide is ferric oxide.
 5. The filter according to claim 1, whereinthe metal oxide is in the form of particles less than 1 μm in size. 6.The filter according to claim 5, wherein the metal oxide is in the formof particles less than 500 nm in size.
 7. The filter according to claim1, wherein the metal oxide constitutes a maximum of 5% by weight of thepolymer material.
 8. The filter according to claim 1, wherein the metaloxide constitutes a minimum of 0.1% by weight of the polymer material.9. The filter according to claim 1, wherein metal oxide constitutes from0.25% to 3% by weight of the polymer material.
 10. The filter accordingto claim 1, wherein the polymer matrix is selected from the groupconsisting of polyacrylics, polyesters, polyamides, polyolefins,fluorocarbon fibres and polyphenylene polymers, copolymers andderivatives thereof and mixtures thereof.
 11. The filter according toclaim 1, wherein the polymer matrix is a polyacrylonitrile-containingpolymer, or a derivative thereof.
 12. The filter according to claim 1,wherein the polymer matrix is polyacrylonitrile (PAN) or a copolymer ofacrylonitrile with one or more other monomers.
 13. The filter accordingto claim 11, wherein the polymer matrix is a copolymer of acrylonitrileand methacrylonitrile.
 14. The filter according to claim claim 11,wherein material comprises at least 85% acrylonitrile.
 15. The filteraccording to claim 13, wherein the polymer matrix comprises at least 95%acrylonitrile by weight.
 16. The filter according to claim 13, whereinthe polymer material comprises at least 99% acrylonitrile by weight. 17.The filter according to claim 11, wherein the co-monomer content rangesfrom 0 to 10%, by weight of the polymer material.
 18. The filteraccording to claim 1, wherein the average molecular weight of thepolymer is between 100,000 and 300,000.
 19. The filter according toclaim 1, wherein the polymer material is in the form of filaments,fibres, yarn, web, fabric, mat, film or sheet, or a combination of theseforms.
 20. The filter according to claim 19, wherein the polymermaterial is in the form of a flexible fabric, web or mat.
 21. A filterformed at least partly from a polymer material comprising: a polymermatrix, and a metal oxide, the metal oxide being incorporated into thepolymer matrix in an amount to provide a polymer material that has acharacteristic shrinkage rate of 0.2% or less per 1000 test hours in aflue gas environment of approximately 125° C.
 22. A filter formed atleast partly from a polymer material comprising: a polymer matrix, and ametal oxide, the metal oxide being incorporated into the polymer matrixin an amount to provide a polymer material that has a characteristicshrinkage rate that is 40% or less compared to the characteristicshrinkage rage of the polymer material without the metal oxide measuredover a test period of 1000 hours in a flue gas environment atapproximately 125° C.
 23. Use of a polymer material comprising: apolymer matrix selected from the group consisting of polyacrylics,polyesters, polyamides, polyolefins, polyaramids, fluorocarbon fibresand polyphenylene polymers, co-polymers and derivatives thereof, andmixtures thereof, and a metal oxide, in an environment having atemperature of at least 100° C.
 24. Use according to claim 23, whereinthe environment is a flue gas environment.
 25. Use of a polymer materialcomprising a polymer matrix and a metal oxide in the manufacture of afilter.
 26. A method of forming a filter, the method comprising thesteps of: providing a polymer matrix; incorporating particulate metaloxide into the polymer matrix; casting or spinning the product into afilm or a fibre; and forming the film or fibre into a filter material.